The present invention relates generally to scheduling transmissions to and from user terminals over shared channels in a mobile communications system and, more particularly, to a coordinated signaling scheme supporting both uplink and downlink transmissions over shared channels designed to reduce current drain in a user terminal.
In mobile broadband communication systems, a plurality of user terminals may share an uplink and/or downlink channel. For example, the developing standard for mobile broadband systems known as Long Term Evolution (LTE) provides two shared channels; one for uplink transmissions and one for downlink transmissions. A scheduler at the base station, referred to in the LTE standard as an Evolved Node B (eNB), determines which user terminals shall have use of the shared channels at any given scheduling interval and which resources the user terminals shall use. Scheduling decisions are typically made once per subframe. Because LTE uses Orthogonal Frequency Division Multiplexing (OFDM), the radio resources may be divided in the frequency domain among two or more user terminals at any given time.
For downlink communications, the user terminals report channel conditions to the scheduler. The scheduler determines which user terminals to serve based on the reported channel conditions and other factors such as buffer levels, quality of service (QoS) requirements, and fairness considerations. Data is transmitted to the user terminals on the Physical Downlink Shared Channel; (PDSCH). The identities of the user terminals scheduled to receive data in a given subframe are transmitted to the user terminals in a scheduling message transmitted on a shared downlink control channel, referred to in the LTE standard as the Physical Downlink Control channel (PDCCH). Thus, the user terminals must monitor the PDCCH to determine when they are being scheduled to receive data on the PDSCH. After receiving data on the PDSCH, the user terminal transmits an acknowledgement (ACK/NACK) to the base station on the Physical Uplink Control Channel (PUCCH) to indicate whether the data was successfully decoded. In the case of a NACK, the user terminal continues monitoring the PDCCH for scheduling information relating to the retransmission.
For uplink communications, the LTE standard provides a scheduling request mechanism to enable the user terminal to request resources for uplink transmissions. To request resources, the user terminal transmits a scheduling request (SR) on the PUCCH to the base station, indicating that the user terminal has new data to send. The user terminal then monitors the PDCCH while it waits for an uplink grant (UL-G). Once the UL-G is received, the user terminal transmits data on the assigned resources. Because the SR contains no information about the resource requirements of the user terminal, the user terminal attaches a buffer status report (BSR) to the first uplink transmission. After transmitting the data to the base station, the user terminal then monitors the Physical HARQ Indicator Channel (PHICH) for an ACK/NACK of the uplink transmission.
The LTE standards supports an operating mode known as discontinuous reception (DRX) to enable the user terminal to conserve power by turning off some of its radio circuitry. The DRX mode is defined by a DRX cycle, an on-duration, and an inactivity timer. The user terminal wakes up and monitors the PDCCH at the beginning of every DRX cycle for a period equal to the on-duration. If no scheduling message directed to the user terminal is received, the user terminal falls asleep until the beginning of the next DRX cycle. When the user terminal receives a scheduling assignment from the base station, it starts the inactivity timer and continues to monitor the PDCCH until the timer expires.
While the DRX operating mode is intended to save battery power, the amount of actual savings may be disappointing. The lack of coordination between the uplink and downlink scheduling algorithms may prevent, or significantly reduce, potential power savings from DRX operation. When the user terminal is operating in DRX mode, the user terminal may still need to turn on its receiver for uplink operations. For example, when the user terminal requests resources for uplink transmissions, the user terminal turns on its receiver to monitor the PDCCH while waiting for an uplink grant. Further, HARQ operations for uplink communications overrides DRX operations and the user terminal will turn on its receiver to listen for an ACK/NACK message from the base station. Furthermore, when retransmissions are required for previously-sent downlink transmissions, the user terminal must turn on its receiver to listen for downlink scheduling messages.
Because long battery life is a major selling point of mobile communication devices, there is much interest in finding ways to reduce power consumption LTE terminals and other future generation user terminals.